This invention relates to a hold-type display device such as a liquid crystal display, an electroluminescence (EL) display or a liquid crystal on silicon display (LCOS), or in particular, to a display device and a display system suitable for displaying dynamic images.
The display devices can be classified roughly into an impulse-type display device and a hold-type display device from the viewpoint of dynamic image display. In the impulse-type display device such as the cathode-ray tube, the brightness of the pixels scanned is increased only during the scanning period and decreased immediately after scanning. The hold-type display device such as the liquid crystal display, on the other hand, continues to hold the brightness based on the display data until the next scanning period.
According to US Patent No. 2004/0155847, one frame period is divided into first and second periods, and the pixel data to be written in the pixels during the frame period are written concentratedly in the first period. In the process, the value written into the pixels is increased to the double of the image data value not to decrease the brightness of the image as a whole, and only in the case where the double value exceeds a displayable range, the residual pixel data is written in the second period. In this way, the change in display brightness approaches that of the impulse-type display device thereby to improve the visual recognizability of the dynamic image.
According to JP-A-2002-215111, on the other hand, a frame memory unit stores one frame of input video signal, and a frame rate conversion signal generating unit generates a clock signal, a horizontal sync signal and a vertical sync signal with the video signal frame rate converted in a magnification of 3 or more from the clock signal, horizontal sync signal and the vertical sync signal, respectively, synchronized with the input video signal. Also, the output signal of the frame memory unit and the video signal fixed at black level are switched and output by a video signal switching unit based on the switching signal output from a switching signal generating unit, so that the display period of one frame image is shortened arbitrarily.
In similar fashion, JP-A-2004-317928 discloses a liquid crystal display device comprising a frequency changing circuit 11 for outputting each input frame four times at a rate four times higher and a liquid crystal display element 15 for displaying each frame output 2N times, wherein the brightness level of every other conversion frame is converted to a lower level than the brightness level of the remaining every other conversion frame and supplied to the liquid crystal display element, thereby reducing the motion blur of the dynamic image caused at the time of each frame change.
The above-mentioned drive method is called the n-fold rate impulse-type drive below in this specification.
In the display device including an n-fold rate impulse-type drive described in US Patent No. 2004/0155847, the input display data is required to be plotted n times, i.e. the output display data is required to be rewritten within one frame period, which in turn requires an n-fold rate increasing means to increase the input display data to a rate n times higher. A frame memory is required, however, for the n-fold rate increase. Specifically, the cost of the display device having the n-fold rate impulse-type drive is higher than that of the normal display device by the amount of the frame memory. According to US Patent No. 2004/0155847, the first period data and the second period data are generated in the display device, and therefore, no consideration is given to the fact that in the case where the data for the first and second periods are generated outside the display device and input sequentially to the display device, the first period data and the second period data cannot be discriminated from each other.